Is ModSubPrc an @Entity? If so, be sure there is a hashCode() and equals() method defined, so the converter has a way to compare.

Also, injecting modSubPrc might be interfering with the value binding of the selectOneMenu.

A third thing, which is a style matter: I generally only have accessors/mutators for submitted values and outject the lists, for controls like selectOneMenu, to conversation scope. Cuts a bit of noise from the backing bean.